@speedbone@yvan it is. But officially unsupported, so it will break if you don't repatch it after upgrades. That said, I'm using it. But the "ro" option on FreeBSD is officially supported.
@yvan@stefano read-only root on OpenBSD is easy: . mount_mfs /dev with size 20m and -P option pointing to a copy of /dev; needed for writable device files; copy of /dev in any dir can be created with 'sh /dev/MAKEDEV all' . Writable /etc/random.seed is needed at shutdown/reboot. So, make it a symlink to /var/db/random.seed or /var/run/random.seed. . Test above and set mount / mount option in fstab to ro.